Attention is being given to a slim type display apparatus using an organic EL as a self-luminous high-brightness display. This display apparatus, being self-luminous, requires no backlight unlike a liquid crystal display apparatus. The whole display panel can be slimmed to approximately 1 to 2 mm, thus attaining reductions in size and weight. In addition, there are advantages such as no limitation of a viewing angle, high response speed, high brightness, high contrast and low power consumption. Accordingly, the organic EL display is taken as a promising candidate of a next-generation display. The organic EL display has been applied to a small display for a mobile apparatus (portable information tools) such as a digital camera and a cellular phone. Furthermore, it is expected that the display will be applied to a middle- and large-sized display such as a PC monitor and a TV in the near future. An optimum display image is required to be realized in various use environments from dark places such as in rooms to open light places under the sun because mobile apparatuses can be easily carried regardless of indoor or outdoor. Further, for PC monitors and TVs as well, an optimum display image is required to be realized because they are used under various environments by users.
In a display apparatus such as CRT, liquid crystal or organic EL type, refresh operation of rewriting a video frame to be displayed a several tens of times per second is performed. The frame rewriting frequency is referred to as a refresh rate. When the fresh rate is low, flicker occurs. Accordingly, the refresh rate of these display apparatuses is usually a frequency (60 Hz) at which no flicker occurs. The liquid crystal display apparatus restrains flicker generation by a drive method of reversing a polarity of a voltage to be applied to a pixel electrode, for every frame with respect to a reference voltage, reversing a polarity for every horizontal pixel line or reversing a polarity for every display pixel.
An organic EL display apparatus uses a self-luminous display element for each pixel and emits light by passing electric current through respective light emitting elements to display an image. The brightness of a display screen can be set according to light-emission period occupied in one frame or light emission intensity. A difference between light emission (light portion) and non-light emission (dark portion) is made visible by a user, depending upon a frequency of light emission or a rate (duty ratio) of light-emission period to non-light-emission period in one frame. The difference is recognized as flicker of the display screen. Accordingly, even if display is made with the refresh rate of an image to be displayed being 60 Hz, the display screen flickers, depending upon duty ratio and hence display quality is degraded.
Increasing the refresh rate of a video to be displayed will generate no flicker. However, the operating speed of a drive circuit must be increased and, power consumption increases, and thereby members to be used, such as electronic parts and a drive circuit need a major change.
Japanese Patent Application Laid-Open No. 2006-030516 discloses a drive method of restraining flicker without increasing refresh rate in spite of a duty drive system which controls the brightness of a display screen according to the duty ratio of a light-emission period. This is a drive method which restrains flicker generation by dividing one frame into a plurality of sub frames by light-emission control and emitting light for only light-emission period corresponding to the duty ratio at the respective subframes.
As a drive method of making a gradation display by similar impulse operation, U.S. Pat. No. 6,587,086 discloses a sub field method in its specification. Multi-gradation display is made by dividing one field corresponding to one image into a plurality of subfields, setting a rate of a light emission maintenance period in the respective subfields to power of two and combining these subfields. By setting rates among light-emission maintenance periods of eight subfields SF1, SF2, . . . , SF8 to 1:2:4:8:16:32:64:128, respectively, 256 gradations can be attained in combinations of subfields.
As described in detail below, when impulse operation is performed at a fixed duty ratio with an active matrix type display apparatus, a lighting area moves with a fixed width from the upper to the lower portions of a screen, and a percentage of a lighting area to a non-lighting area occupied in the whole screen changes. Thus, a total current amount flowing into the display area changes with time, and the current change causes a change in power supply voltage because power supply impedance is not completely zero.
Upon the change in power supply voltage, the brightness of a screen changes as the whole and therefore a relationship between a change in power supply voltage and movement of the lighting area generates a phenomenon that a specific area of the screen is darker than the other areas. This brightness unevenness occurs fixedly in a specific area of the screen and therefore such a state cannot be eliminated even if an impulse operation frequency is increased, which degrades image quality due to a cause different from flicker.
Next, this phenomenon will be described in detail below. In the following description, the one field period is taken as a minimum unit period required until the next image data is input after the data required to display one image is input into a pixel for light emission. A period from completion of a row scanning period during a field period to completion of a field period is taken as a vertical blanking period.
FIG. 19 is a view illustrating a change in total current amount flowing into a display area while driving to provide a partially non-light-emission period, hereinafter referred to as “duty driving” in one field period (a total of one time vertical scanning period and vertical flyback time). TS signal is a light-emission control signal at a leading row in a display area and, if the signal is in Hi, light emission is made and if in low, non-light emission is made.
In the display area, there are provided pixels in a two-dimensional manner of m rows and n columns, where m and n are a natural number, respectively. Data is sequentially written into the pixels and a signal for selecting a writing row is scanned by m rows and TS signal is also sequentially scanned by respective rows.
The light emission pattern in FIG. 19 indicates impulse operation timing in a plurality of rows at uniform intervals within a display area. The leading row of the display area has the same light emission pattern as TS signal illustrated on the top of FIG. 19. Respective rows arranged at constant intervals delay light emission start by a scanning period of the interval compared with the leading row. The light emission patterns temporally-shifted in FIG. 19 illustrate light-emission periods of the leading row and a row following the leading row.
A broken line at the bottom of FIG. 19 shows a virtual light emission pattern of a “non-display area”. Vertical scanning where light-emission periods are sequentially shifted is extended to a vertical blanking period. Assuming an area virtually scanned during the period, such an area is referred to as a “non-display area”. No actually scanned or light emission rows exist during this period.
ΣI in FIG. 19 illustrates a sum of currents flowing into elements which are emitting light, that is, a total current amount (ΣI) flowing into a display area.
As illustrated in FIG. 19, ΣI changes depending on time. Changes in ΣI will be described in detail below.
FIG. 22 illustrates movement states (diagonal light and dark pattern) of a lighting area and a non-lighting area while a light emission area is moving from the top to the bottom of a display area, and brightness distribution (a graph below the light and dark pattern). FIG. 19 illustrates that the number of times of light emission is one within one field period, while FIG. 22 illustrates that the number of times is two within one field period.
A light and dark light emission pattern 101 illustrates that a position in a row scanning direction (a vertical direction in display area) is indicated in the horizontal direction and a time is indicated in a vertical direction. A white portion refers to light emission and a black portion refers to non-light emission. A graph of a light emission pattern in FIG. 19 corresponds to a light emission pattern 101 in FIG. 22 vertically cut.
A time change 102 of a total current ΣI is illustrated on the right of the light emission pattern 101. The vertical axis indicates time, which meets the time in the light emission pattern 101 within a display area. ΣI alternately repeats a period 105 when a large value is obtained and a period 106 when a small value is obtained. Reference numeral 103 denotes a vertical blanking period.
The number of lighting rows and the number of non-lighting rows are constant along a vertical direction (horizontal axis of 101) within a display area for a while after the leading row (left end of 101) of the display area changes from ON to OFF, and ΣI as well takes a constant value. During this period 105, two bands of lighting rows are moving from the top to the bottom of the display area. The number of lighting rows is more than that of non-lighting rows and a difference thereof is equal to the number of virtual scans within a vertical blanking period.
Then, with the leading row of the display area being OFF, when the final row changes from OFF to ON, subsequently the number of lighting rows decreases and the number of non-lighting rows increases. Thus, ΣI decreases. A decrease in the number of lighting rows and an increase in the number of non-lighting rows change with time at a constant rate and therefore ΣI shows a linear change with respect to time.
When the leading row enters a lighting period, the number of lighting rows and the number of non-lighting rows become constant again, respectively. This period 106 is a period when two bands of non-lighting rows move from the top to the bottom of the display area and therefore the number of lighting rows is less and the number of non-lighting rows is more than during the period 105. (The difference is still equal to the virtual number of scans during the vertical blanking period.) Accordingly, ΣI value is smaller than during the period 105.
Then, after the leading row of the display area maintains ON and the final row shifts to OFF, the number of lighting rows increases and the number of non-lighting rows decreases. Hence, ΣI increases.
The above is one cycle of ΣI time change. When a vertical blanking period exists in this way, a difference between lighting rows and non-lighting rows within a display area changes. This is a possible cause of a ΣI change.
A power supply has power source impedance of the apparatus's own. Accordingly, when ΣI changes, power supply voltage drops according to the product of the power source impedance and ΣI, thus causing a power supply change.
When power supply voltage drops, a change in brightness is caused. One of the possible causes is a current-voltage characteristic of a driving transistor. FIG. 20 illustrates Vds−Ids characteristic of driving TFT (Thin Film Transistor). In a case where a saturated area of TFT is used to drive a light emitting element, a voltage drop causes a current decrease by an early effect. Thus, a current flowing into a self-luminous element decreases to degrade brightness.
Another possible cause of brightness change is a current-voltage characteristic of the self-luminous element. FIG. 21 is a voltage-current characteristic of a typical organic EL device. When an applied voltage to a light emitting element such as an organic EL decreases, electric current also decreases to degrade brightness.
Depending upon configuration of a pixel circuit, there may be the case where an electric current flowing into the self-luminous element increases when a power supply drops, so that brightness may be enhanced, but here a case of a circuit configuration which decreases brightness with power supply drop is taken.
The lower portion of the light emission pattern 101 in FIG. 22 illustrates how brightness changes are seen on a display apparatus.
During a period of reference numeral 105, a total current amount is large and a power supply is in a dropping state and therefore the brightness of a light emission position during this period is low. During a period of reference numeral 106, a total current amount is small and a power supply is not in a dropping state and therefore the brightness of a light emission position during this period is higher than those of any other positions. The result obtained when these changes in brightness are integrated within a field period is denoted by a reference numeral 104. The time changes of ΣI by a light emission pattern are synchronous with movement of the light emission pattern and therefore brightness degrades at a specific position in a row scanning direction and looks like a light and dark pattern the position of which is fixed on the display screen. Such unevenness of brightness degrades image quality.
The degree of the brightness changes is determined by integration of a plurality of factors such as the magnitude of power source impedance, sensitivity of a pixel circuit against voltage drop, influence of TFT characteristics and efficiency of a self-luminous element.
FIG. 23 illustrates ΣI of a display apparatus light-emitted in a light emission pattern of FIG. 22 and time changes in light emission brightness of respective positions (1) to (4). Specifically, FIG. 23 illustrates light-emission control signal TS, total current amount ΣI flowing into a display area depending upon light emission timings, light emission timing of positions (1) to (4) in a specific row within a display area, then brightness and respective time changes. For light emission timing and brightness, a Low level indicates OFF, a High level indicates light emission and a Medium level indicates slightly dark light emission, and a slanting line illustrates gradually changing brightness.
Position (1) illustrates a state of light emission at the leading row of a display area and is almost the same light emission pattern as TS signal. Positions (2) to (4) illustrates a state of light emission at a position downwardly shifted by each ¼ in the vertical direction of the display area from Position (1), respectively. As a row is shifted by row scanning, light emission start of TS signal delays by the time and, as illustrated, a light emission timing changes depending upon row. With attention focused on changes in ΣI, a small ΣI period 106 in FIG. 22 corresponds to periods P1, P2, P1′, P2′ in FIG. 23.
At Position (1) light emission starts immediately after field period start and, as illustrated in FIG. 22, a first half (P1 period) of the light-emission period is a period at which ΣI is small and constant and power supply voltage is kept high and therefore light emission is made with high brightness. However, with an increase in ΣI from midway, power supply voltage drops, and therefore light emission brightness decreases. A second light emission is made in the same light emission pattern.
At position (2) light emission starts at a high ΣI position and therefore light emission is made with slightly low brightness. Subsequently, brightness rises a little with lowering of ΣI. The second light emission is made in the same light emission pattern.
Light emission start timings of Position (3) and Position (4) delay by a ½ field period from those of Position (1) and Position (2), but the light emission pattern is exactly the same.
At Position (2) and Position (4), rising ΣI changes and light-emission period synchronize with each other and therefore there is hardly a period of high light emission. Accordingly, a large difference in light emission amount at respective rows integrated in a certain period (e.g. one field period) occurs and a brightness change occurs in a row direction within the display area, thus degrading image quality.